(1) Field of the Invention
This invention relates to unidirectional spiral antennae and more particularly to radiation absorber systems associated therewith.
(2) Description of the Prior Art
It is well-known in the prior art to employ a reflector cavity with a spiral antenna so as to render the antenna unidirectional, cf. U.S. Pat. Nos. 3,192,531; 3,358,288; 3,441,937; 3,555,554; 3,686,674; 3,781,898; 3,820,117 and 3,969,732, for example.
It is also well-known in the prior art to provide radiation absorber systems in such reflector cavities. Thus, for example, a plurality of triangular-shaped absorber card attenuators are positioned radially and symmetrically in a radial finlike member in the reflector cavity of the spiral antenna of U.S. Pat. No. 3,192,531. A microwave absorber liner either in layer or discrete member form is used to cover the entire peripheral and bottom inner walls of the cylindrical reflector cavities of the antennae of U.S. Pat. Nos. 3,441,937 and 3,820,117. In a similar manner, the cavity chamber floor of the antenna of U.S. Pat. No. 3,781,898 is provided with a discrete absorber member. In U.S. Pat. Nos. 3,555,554 and 3,686,674 one or more spaced concentric radiation absorber members are disposed in the reflector cavities disclosed therein.
As used herein, the radiation absorption density characteristic refers to the density of the absorber material particles used to absorb the rf radiation. Lossy materials suitable for this purpose are, for example, Eccosorb .RTM. SF-11, manufactured by Emmerson and Cumming of Camden, Massachusetts, dispersed in a silicon rubber matrix, cf. U.S. Pat. No. 3,441,937; graphite particles impregnated in a dielectric sheet, cf. U.S. Pat. No. 3,555,554; ferrite particles suspended in an epoxy, cf. "Casting Terminators For High-Frequency Antennae," S. M. Cole, IBM .RTM. Technical Disclosure Bulletin, Vol. 18, No. 8, January 1976, page 2475. Also as used herein, laterally transverse refers to a direction substantially normal to the associated unidirectional axis of the antenna and longitudinal refers to a direction substantially parallel to it.
In each of the absorber systems of the aforedescribed prior art, the laterally or radially transverse radiation absorption density characteristic of each of the individual absorber members thereof is the same, i.e. uniform, and was equal to its longitudinal radiation absorption density characteristic. Hence, both the lateral and longitudinal density characteristics of the absorber system were the same and invariable for a given member. As a result, the control of the associated antenna's absolute gain versus frequency characteristic was limited to varying the dimension parameters of the various absorber components and/or of the cavity.
In another prior art device, the radiation absorber system is composed of plural radiation absorber members which are vertically, i.e. longitudinally, stacked in the reflector cavity of the antenna. While the absorber components of this particular absorber system have different radiation absorption density characteristics, the lateral radiation absorption density characteristic for any particular component and/or of the composite system is still uniform. That is to say, in this particular prior art device while the composite longitudinal radiation absorption density characteristic of the absorber system was variable, its corresponding lateral characteristic was uniform. Thus, this particular system was also limited in the manner in which it could control the antenna pattern, i.e. its absolute gain versus frequency characteristic. In addition, those members remote from the cavity opening were not readily accessible for removal, replacement, etc. without first removing the intervening members and thereby increased the time and cost in maintaining, servicing, etc. the antenna. Moreover, where the individual absorber members of this particular device have to be arranged in the cavity in accordance with a predetermined radiation absorption density characteristic sequence in the vertical direction, it was possible to assemble the individual members, all of which being of the same diameter, in an undesirable density characteristic sequence.